Most plastic articles are subject to an increase in yellowness after aging under an ultra violet source. For example, polycarbonate has increased Y.I., yellowness index as measured by ASTM D 1925-63T, after exposure to ultra violet light. The usual method of inhibiting the yellowing of polymers caused by ultraviolet light is the addition of an additive to the bulk material prior to extrusion. This method, known as bulk stabilization, is generally quite successful in inhibiting the degradation of the polymer.
Polyarylates are well known to have severe yellowing problems when exposed to ultraviolet light. Although not totally understood, it is thought that this yellowing is due to a photo-Fries rearrangement occuring at the ester bond linkage to an aryl group. This ultra violet light promoted reaction results in a rearrangement of the type illustrated below ##STR2##
This type of degradation leaves a layer on the surface of the polyarylate which is the product of the photo-Fries rearrangement. If such product is colored, the plastic article surface will quite rapidly assume that color. Since this is a surface phenomenon, a point is reached, usually after a relatively short period of exposure to the sun, wherein the rate of yellowing of the underlying polymer is significantly reduced. When such mechanism of degradation is occurring, bulk stabilization provides an insufficient amount of additive at the surface of the article to inhibit the yellowing. A different method of stabilization is required.
Aromatic copolyestercarbonates have similar problems. A yellow surface coloring occurs after exposure to an ultraviolet source. Attempts by the inventor to impregnate the surface of the copolyestercarbonate with seemingly appropriate ultraviolet inhibitors have not proved successful.
A special class of ultraviolet inhibitors has been discovered. This class does provide very good ultraviolet stability when impregnated upon the surface of an article prepared from aromatic copolyestercarbonate.